Lawn mower having selectively drivable wheels

ABSTRACT

A lawn mower having selectively drivable wheels, includes dirigible front drive wheels, a pair of right and left rear drive wheels, a body frame, an engine supported by the body frame, a mower unit vertically movably supported by the body frame forwardly of the rear drive wheels, a change speed device for changing a speed of drive from the engine, a front wheel driving device for transmitting drive from the change speed device to the front wheels, a left rear wheel driving device for transmitting drive from the change speed device to a right axle of the left rear wheel, a right rear wheel driving device for transmitting drive from the change speed device to a left axle of the left rear wheel, and rear wheel clutches associated with the left rear wheel driving device and the right rear wheel driving device, respectively, for selectively connecting and disconnecting drive. The rear wheel clutches are operable by an interlocking mechanism in response to a displacement to the front drive wheels from a straight running posture, to disconnect drive transmission to one of the rear drive wheels moving along an inner track of turning of the vehicle body. The interlocking mechanism has a link mechanism operatively connecting a steering mechanism for steering the front drive wheels to each of the rear wheel clutches, and extending over the mower unit.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a lawn mower having selectivelydrivable wheels.

[0003] 2. Description of the Related Art

[0004] A conventional lawn mower has freely rotatable dirigible frontwheels, and rear drive wheels. A rear wheel differential mechanism isdisposed on a drive transmission line from the engine to the rearwheels. When making a turn, the right and left rear drive wheels aredriven at different speeds. Another type of lawn mower has dirigiblefront drive wheels, and rear drive wheels. A front wheel differentialmechanism is disposed on a drive transmission line from the engine tothe front wheels, while a rear wheel differential mechanism is disposedon a drive transmission line from the engine to the rear wheels. Whenmaking a turn, the right and left front and rear drive wheels are drivenat different speeds through the two differential mechanisms.

[0005] A differential mechanism can absorb a difference in rotationbetween right and left wheels occurring in time of making a turn, and isthus used as an indispensable component of an automobile or the like.However, where drive is transmitted to drive wheels through adifferential mechanism, a difference in rotation between the right andleft drive wheels is caused also by loads applied from the ground to thedrive wheels in certain surface conditions. As a result, the vehiclewill lose its straight running performance. This problem is serious fora lawn mower that runs on lawns or meadows in bad ground conditions,compared with an automobile that runs on paved roads. Further, when asudden decrease occurs in the load applied from a lawn to one of therear drive wheels, that rear drive wheel suddenly slips and rotatesfast, thereby marring the lawn.

[0006] Thus, a conventional tractor has, instead of using a differentialmechanism, a one-way clutch mechanism in each pf drive transmissionlines to right and left drive wheels. The drive wheels are switchable byaction of the one-way clutch mechanisms. Such a tractor is known fromBritish Patent Publication GB2335716 or U.S. Pat. No. 4,733,743, forexample. The known tractor achieves a smooth turning performance,without a differential mechanism, by breaking drive transmission to arear drive wheel rotating excessively as it moves along an outer turningtrack. However, even a slight difference in rotation between the rightand left rear wheels causes operation of a clutch mechanism. Thisreduces durability of the clutch mechanisms, and noise produced by theclutch mechanisms is far from negligible.

[0007] Further, a rice planting machine must move with a high straightrunning performance and sharp turning performance in a paddy field invery difficult surface conditions. The rice planting machine employs atechnique of providing a clutch in each of drive transmission lines toright and left rear drive wheels, for breaking drive transmission to oneof the rear drive wheels moving along an inner track when the machinemakes a turn. Such rice planting machines are disclosed in JapanesePatent Publications (Unexamined) Nos. 2001-88731 and 1997-220946, forexample.

[0008] A different type of rice planting machine is known from JapanesePatent Publication (Unexamined) No. 2001-278102. This rice plantingmachine has a differential mechanism mounted on a drive transmissionline to right and left rear drive wheels, and clutches for the reardrive wheels. In time of making a turn, the differential mechanism islocked, and drive transmission to one of the rear drive wheels movingalong an inner track is broken.

[0009] A rice planting machine is a special vehicle that runs in a ricefield. Its body is relatively simple, and has a space under a bodyframe. It is therefore relatively easy to provide a control mechanismsuch as clutches for the right and left rear wheels.

[0010] On the other hand, a lawn mower needs a large mower unitvertically movably suspended under a body frame. It is difficult toprovide a control mechanism for switching drive wheels. Thus, nomechanism has been employed for operating clutches for right and leftrear wheels in response to steering of dirigible wheels.

SUMMARY OF THE INVENTION

[0011] The object of this invention is to provide a lawn mower havingwheels selectively drivable in response to a steering operation, toachieve an excellent straight running performance and to be capable ofsmooth turning without marring ground surfaces.

[0012] The above object is fulfilled, according to this invention, by alawn mower having selectively drivable wheels, comprising dirigiblefront drive wheels, a pair of right and left rear drive wheels, a bodyframe, an engine supported by the body frame, a mower unit verticallymovably supported by the body frame forwardly of the rear drive wheels,a change speed device for changing a speed of drive from the engine, afront wheel driving device for transmitting drive from the change speeddevice to the front wheels, a left rear wheel driving device fortransmitting drive from the change speed device to a right axle of theleft rear wheel, a right rear wheel driving device for transmittingdrive from the change speed device to a left axle of the left rearwheel, rear wheel clutches associated with the left rear wheel drivingdevice and the right rear wheel driving device, respectively, forselectively connecting and disconnecting drive, and an interlockingmechanism for operating the rear wheel clutches in response to adisplacement to the front drive wheels from a straight running posture,to disconnect drive transmission to one of the rear drive wheels movingalong an inner track of turning of the vehicle body, the interlockingmechanism having a link mechanism operatively connecting a steeringmechanism for steering the front drive wheels to each of the rear wheelclutches, and extending over the mower unit.

[0013] The interlocking mechanism is provided to disengage the rearwheel clutch corresponding to the rear drive wheel moving along an innertrack of turning of the lawn mower in response to a displacement of thefront drive wheels from the straight running posture to a steeringposture. As a result, the lawn mower makes a turn by driving only theoutward rear drive wheel, with the inward rear drive wheel in freerotation. This causes the outward rear drive wheel reliably and steadilywhile achieving a smooth turn. The question of space may be settled byproviding a link mechanism operatively connecting the steering mechanismfor steering the front drive wheels to each of the rear wheel clutches,and extending over the mower unit.

[0014] In time of straight running, the right and left clutches are bothengaged and the right and left rear drive wheels are, so to speak,directly connected to each other through the drive transmitting line.Thus, a steady straight running state may be maintained even in badsurface conditions. The lawn mower according to this invention has anexcellent straight running performance, and yet is capable of smoothturns with little chance of marring a ground surface.

[0015] Where the rear wheel clutches are friction clutches, drivetransmission to the rear drive wheels may be connected and disconnectedsmoothly. This will also reduce noise generated in time of clutchoperation.

[0016] Where the lawn mower has a grass catcher disposed in a rearposition of the body frame, and a grass collecting duct for transmittinggrass clippings cut by the mower unit rearward to the grass catcher, thegrass collecting duct may be disposed to pass between the right axle andthe left axle. The change speed device may be disposed laterally thegrass collecting duct between the right axle and the left axle. Thisarrangement clears up the question of space with ease.

[0017] The invention provides also a lawn mower having dirigible frontnon-drive wheels, and a pair of right and left rear drive wheels. Then,the lawn mower may advantageously comprise a rear wheel differentialmechanism with a differential lock function, rear wheel clutches forselectively connecting and disconnecting drive to the respective rearwheels, and an interlocking mechanism for operating the rear wheeldifferential mechanism and the rear wheel clutches in response to adisplacement to the front wheels from a straight running posture. Torealize an excellent straight running performance and a turningperformance with little chance of marring a ground surface, theinterlocking mechanism operates the rear wheel differential mechanismand the rear wheel clutches such that, when the front wheels are at asmall steering angle, the rear wheel differential mechanism is unlockedand the rear wheel clutches are both engaged, and when the front wheelsare at a large steering angle, the rear wheel differential mechanism islocked and one of the rear wheel clutches associated with one of therear drive wheels moving along an inner track of turning of the vehiclebody is disengaged.

[0018] In another aspect of the invention, a lawn mower has dirigiblefront drive wheels for receiving drive through a front wheeldifferential mechanism, and a pair of right and left rear drive wheelsfor receiving drive through a rear wheel center clutch and respectiverear wheel clutches. Then, the interlocking mechanism may operate therear wheel center clutch and the rear wheel clutches such that, when thefront wheels are at a small steering angle, the rear wheel center clutchis disengaged, and when the front wheels are at a large steering angle,the rear wheel center clutch is engaged and one of the rear wheelclutches associated with one of the rear drive wheels moving along aninner track of turning of the vehicle body is disengaged.

[0019] In a further aspect of the invention, a lawn mower has dirigiblefront drive wheels for receiving drive through a front wheeldifferential mechanism, and a pair of right and left rear drive wheelsfor receiving drive through a rear wheel differential mechanism with adifferential lock function and respective rear wheel clutches. Then, theinterlocking mechanism may operate the front wheel center clutch, therear wheel differential mechanism and the rear wheel clutches such that,when the front wheels are at a small steering angle, the front wheelcenter clutch is disengaged, the rear wheel differential mechanism isunlocked and the rear wheel clutches are both engaged, and when thefront wheels are at a large steering angle, the front wheel centerclutch is engaged, the rear wheel differential mechanism is locked andone of the rear wheel clutches associated with one of the rear drivewheels moving along an inner track of turning of the vehicle body isdisengaged.

[0020] Other features and advantages of this invention will be apparentfrom the following description of the embodiments to be taken withreference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a side elevation of a lawn mower in a first embodimentof this invention;

[0022]FIG. 2 is a schematic plan view showing a link mechanism in astraight running state of the lawn mower shown in FIG. 1;

[0023]FIG. 3 is a schematic plan view showing the link mechanism in aturning state of the lawn mower shown in FIG. 1;

[0024]FIG. 4 is rear view, partly in section of a portion of the lawnmower shown in FIG. 1;

[0025]FIG. 5 is a schematic view of a drive transmission system;

[0026]FIG. 6 is a schematic plan view showing a link mechanism of a lawnmower in a second embodiment of this invention;

[0027]FIG. 7 is a schematic plan view showing a link mechanism of a lawnmower in a third embodiment of this invention; and

[0028]FIG. 8 is a schematic plan view showing a link mechanism of a lawnmower in a fourth embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] As shown in FIGS. 1 through 3, a lawn mower according to thisinvention has a pair of right and left dirigible front drive wheels 1and a pair of right and left rear drive wheels 2 supporting a body frame3. An engine 4 is mounted on a front portion of the body frame 3, and adriving platform 5 is formed on an upper rear portion of the body frame3. Below the driving platform 5 is a transmission case 6 housing achange speed device including a gear transmission mechanism 6a and astepless transmission 13 (which is an HST here). A mower unit 7 isvertically movably attached to the body frame 3 through a lift mechanism51 in a space between the front drive wheels 1 and rear drive wheels 2.The mower unit 7 is the rear discharge type having a plurality of blades7 a juxtaposed transversely of the body frame 3 for cutting grass andrearwardly discharging grass clippings. In this embodiment, a grasscatcher 53 is attached to the rear end of the body frame 3 through abracket 52. A grass clippings outlet formed in a rear position of themower unit 7 is connected to an opening formed in a front plane of thegrass catcher 53 by a grass collecting duct 54 extending through a spacebetween the right and left rear wheels 2. Thus, grass clippings cut bythe mower unit 7 are transmitted to the grass catcher 53 through thegrass collecting duct 54.

[0030] As shown in FIGS. 2 and 3, a transversely extending cylindricaltransmission case 8 is supported by the body frame 3 to be capable ofrolling about a for fore and aft axis P. A vertical transmission case 9is supported at each of opposite right and left ends of the transversetransmission case 8 to be pivotable about a kingpin axis X. A front axlela of the front drive wheel 1 is supported in a lower end region of eachvertical transmission case 9. The transverse transmission case 8 has afront wheel differential mechanism 19 for the front drive wheels 1. Thetransverse transmission case 8 and right and left vertical transmissioncases 9 contain transmission mechanisms from the front wheeldifferential mechanism 19 to the front axles 1 a of the front drivewheels 1.

[0031] As shown in FIG. 4, a left rear wheel transmission case 10 a anda right rear wheel transmission case 10 b are connected to oppositesides in a lower region of the transmission case 6 for transmittingdrive to the rear wheels 2. The left rear wheel transmission case 10 aand right rear wheel transmission case 10 b are arranged outwardly ofthe body frame 3 and bolted to the body frame 3. The left and right rearwheel 2 have axles 2 a thereof supported by the body frame 3 through theleft rear wheel transmission case 10 a and right rear wheel transmissioncase 10 b.

[0032] In this embodiment, as seen from FIG. 4, the transmission case 6is offset to the left rear wheel 2. A transverse cylindrical case 10 cextends between the transmission case 6 and right rear wheeltransmission case 10 b. The grass collecting duct 54 extends above thecylindrical case 10 c and at a side of the transmission case 6 or, to beprecise, the stepless transmission 13 forming part of the transmissioncase 6.

[0033] The body frame 3 is formed of a pair of right and left band-likeprofile members extending in the fore and aft direction. A large spaceis formed between the right and left band-like profile members. Thechange speed device and grass collecting duct 54 may be arranged in thisspace.

[0034] The drive transmission system of this lawn mower will bedescribed next. As shown in FIGS. 4 and 5, drive is transmitted from theengine 4 to an input shaft 11 of the transmission case 6 through atransmission shaft 12. This input shaft 11 acts also as an input shaftof the stepless transmission 13 disposed rearwardly of the transmissioncase 6. Drive inputted to the stepless transmission 13 is subjected toswitching in rotating direction (forward rotation and backward rotation)and rotating speed by pedal operation not shown. Drive is transmittedfrom the stepless transmission 13 through an output shaft 13 a to a rearwheel transmission gear set 16, and then to a bevel gear mechanism 40.The bevel gear mechanism 40 is connected at an output end thereof toright and left rear wheel transmission shafts 15 extending transverselyfrom opposite sides of the transmission case 6. The right and left rearwheel transmission shafts 15 are disposed at a lower level than the rearwheel axles 2 a. The level difference is absorbed by the rear wheeltransmission gear set 16.

[0035] The output shaft 13 a of the stepless transmission 13 isconnected also to a front wheel output shaft 17 by a front wheeltransmission gear set 18. The front wheel output shaft 17 is connectedto the front wheel differential mechanism 19 through a front wheelintermediate transmission shaft 20.

[0036] That is, the front wheel intermediate transmission shaft 20 andfront wheel differential mechanism 19 constitute a front wheel drivingdevice for transmitting output of the change speed device including thegear transmission mechanism 6 a and stepless transmission 13 to thefront wheels 1. The right and left rear wheel transmission shafts 15 andrear wheel transmission gear set 16 constitute a rear wheel drivingdevice for transmitting output of the change speed device to the rearwheels 2.

[0037] Further, the transmission case 6 includes a front power takeoffshaft 23 interlocked to the input shaft 11 through a gear set and aworking clutch 22. The front power takeoff shaft 23 is connected througha universal joint to a transmission shaft 24 extending to an input shaft7 b of the mower unit 7 for giving drive to the blades 7 a of the mowerunit 7. The input shaft 11 extends through the stepless transmission 13to be connected at a rear end thereof to a rear power takeoff shaft 25.

[0038] In this embodiment, a rear wheel clutch 27 in the form of amultidisk friction clutch is mounted in an intermediate position of eachof the right and left rear wheel transmission shafts 15 forming the rearwheel driving device. The rear wheel clutches 27 are independentlyoperable to connect and disconnect drive transmission from the changespeed device to the right and left rear drive wheels 2.

[0039] This lawn mower has an interlocking mechanism for reversiblydisengaging the rear wheel clutch 27 for the rear drive wheel 2 movingalong an inner track of a turn, in response to a steering operationexceeding a predetermined angle from a straight running posture of thefront drive wheels 1.

[0040] As shown in FIGS. 2 and 3, the interlocking mechanism is in theform of a link mechanism including an interlocking rod 31 connected toone of knuckle arms 30 interlocked through a tie rod 29 of a steeringmechanism for steering the front drive wheels 1 in response to turningof a steering wheel 28 disposed on the driving platform 5. Theinterlocking mechanism further includes a relay link 32 pivotable abouta vertical axis in response to movement of the interlocking rod 31, andcontrol rods 34 operatively connecting opposite ends of the relay link32 to respective control levers 33 of the right and left rear wheelclutches 27. This link mechanism extends over the mower unit 7.

[0041] The operative connection between the interlocking rod 31 andknuckle arm 30 includes a slot 35 acting as a play-accommodating devicefor transmitting a displacement of the knuckle arm 30 to theinterlocking rod 31 only when the steering wheel 28 (and thus theknuckle arm 30) is turned in excess of a predetermined angle from astraight running position. The interlocking rod 31 and knuckle arm 30are connected through a pin inserted in the slot 35.

[0042] The operative connection between each of the right and leftcontrol rods 34 and each control lever 33 includes a slot 36 a thatprovides lost motion for the interlocking relationship between thecontrol rod 34 and the control lever 33 of the rear wheel clutch 27 whenthe latter lies outward in time of the mower making a turn. Further, aplay-accommodating spring 36 b is disposed between the relay link 32 andeach of the right and left control rods 34. The spring 36 b iselastically deformable for allowing steering of the front drive wheels 1following disengagement of the rear wheel clutch 27 lying inward in timeof the mower making a turn.

[0043] With this construction, when the front drive wheels 1 are steeredin excess of the predetermined steering angle (i.e. swing angle ofknuckle arms 30) from the straight running posture, the control rods 34of the link mechanism are displaced to swing one of the control levers33 (the control lever belonging to the rear wheel clutch 27 inward intime of a turn). Consequently, the rear wheel clutch 27 inward in timeof a turn is disengaged.

[0044] This embodiment includes the play-accommodating device (slot 35)which allows the rear wheel clutches 27 to be inoperative until thesteering wheel 28 (and thus the knuckle arm 30) is turned in excess ofthe predetermined angle from the straight running position. Thisplay-accommodating device may be omitted, so that the interlocking rods31 are displaced when the steering wheel 28 is turned only a smallamount from the straight running position.

[0045]FIG. 6 shows a lawn mower having selectively drivable wheels in asecond embodiment of this invention. This embodiment is different fromthe foregoing first embodiment in that the front wheels 1 arenon-driving type dirigible wheels, and that a rear wheel differentialmechanism 140 with a differential lock function is used instead of thebevel gear mechanism 40 for transmitting drive to the right and leftrear wheel transmission shafts 15. The interlocking mechanism operatesnot only the rear wheel clutches 27 but also the rear wheel differentialmechanism 140 in response to a displacement of the front wheels 1 fromthe straight advancement posture to a steering posture.

[0046] The structure of the rear wheel differential mechanism 140 with adifferential lock function is well known, and will not particularly bedescribed here. A lock lever 141 is swingable to lock the differentialmechanism 140, thereby prohibiting its differential operation. Then, thedifferential mechanism 140 acts like a bevel gear mechanism.

[0047] This interlocking mechanism is constructed as a link mechanism asin the first embodiment. Here, the link mechanism further includes alock control rod 142 for operatively connecting the relay link 32 andlock lever 141.

[0048] With this construction, when the front wheels 1 are steered inexcess of a predetermined steering angle (i.e. swing angle of knucklearms 30) from the straight running posture, as in the first embodiment,the control rods 34 of the link mechanism are displaced to swing one ofthe control levers 33 (the control lever belonging to the rear wheelclutch 27 inward in time of a turn). Consequently, the rear wheel clutch27 inward in time of a turn is disengaged to break drive transmission tothe inward rear drive wheel 2. In this embodiment, the lock control rod142 also is displaced at the same time to swing the lock lever 141,thereby locking the differential mechanism 140.

[0049] When the front wheels 1 are maintained in the straight runningposture or steered by an angle not exceeding the predetermined steeringangle (swing angle of knuckle arms 30), the rear wheel differentialmechanism 140 is in unlocked state to be capable of differential action,and the two rear wheel clutches 27 are engaged.

[0050]FIG. 7 shows a lawn mower having selectively drivable wheels in athird embodiment of this invention. This embodiment is different fromthe foregoing first embodiment in that a rear wheel center clutch 240 isadded upstream of the bevel gear mechanism 40 with respect to the drivetransmitting direction for selectively connecting and disconnecting thedrive from the stepless transmission 13. The interlocking mechanismoperates not only the rear wheel clutches 27 but also the rear wheelcenter clutch 240 in response to a displacement of the front wheels 1from the straight advancement posture to a steering posture.

[0051] This interlocking mechanism is constructed as a link mechanism asin the first embodiment. Here, the link mechanism further includes acontrol rod 242 for operatively connecting the relay link 32 and acontrol lever 241 of the rear wheel center clutch 240. This linkmechanism maintains the rear wheel center clutch 240 disengaged when thefront wheels 1 are steered by a small steering angle, and engaged whenthe front wheels 1 are steered by a large steering angle.

[0052] With this construction, when the front wheels 1 are steered inexcess of a predetermined steering angle (i.e. swing angle of knucklearms 30) from the straight running posture, as in the first embodiment,the control rods 34 of the link mechanism are displaced to swing one ofthe control levers 33 (the control lever belonging to the rear wheelclutch 27 inward in time of a turn). Consequently, the rear wheel clutch27 inward in time of a turn is disengaged to break drive transmission tothe inward rear drive wheel 2. In this embodiment, the control rod 242also is displaced at the same time to swing the control lever 241 of therear wheel center clutch 240, thereby engaging the rear wheel centerclutch 240. Since drive is transmitted through the bevel gear mechanism40 to the right and left rear wheel transmission shafts 15, the rearwheel 2 moving along an outer track of a turn is driven. Thus, the lawnmower makes a turn while driving the front wheels and one of the rearwheels.

[0053] When the front wheels 1 are maintained in the straight runningposture or steered by an angle not exceeding the predetermined steeringangle (swing angle of knuckle arms 30), the control rod 242 is notdisplaced, thereby maintaining the rear wheel center clutch 240disengaged. As a result, the lawn mower is propelled only by the frontwheels 1 (front wheel drive).

[0054]FIG. 8 shows a lawn mower having selectively drivable wheels in afourth embodiment of this invention. This embodiment is different fromthe foregoing second embodiment in that the front wheels 1 are drivingtype dirigible wheels as in the first embodiment, and that a front wheelcenter clutch 340 is added upstream of the front wheel differentialmechanism 19 with respect to the drive transmitting direction forselectively connecting and disconnecting the drive to the front wheels1. The interlocking mechanism operates the rear wheel clutches 27, rearwheel differential mechanism 140 and front wheel center clutch 340 inresponse to a displacement of the front wheels 1 from the straightadvancement posture to a steering posture. In this connection, asdistinct from the second embodiment, this interlocking mechanismincludes a control rod 342 connected at one end thereof to the knucklearm 30 by a pin inserted in the slot 35, and at the other end connectedto a control lever 341 of the front wheel center clutch 340.

[0055] This link mechanism maintains the front wheel center clutch 340disengaged when the front wheels 1 are steered by a small steeringangle, and engaged when the front wheels 1 are steered by a largesteering angle.

[0056] With this construction, when the front wheels 1 are steered inexcess of a predetermined steering angle (i.e. swing angle of knucklearms 30) from the straight running posture, as in the first embodiment,the control rods 34 of the link mechanism are displaced to swing one ofthe control levers 33 (the control lever belonging to the rear wheelclutch 27 inward in time of a turn). Consequently, the rear wheel clutch27 inward in time of a turn is disengaged to break drive transmission tothe inward rear drive wheel 2. The lock control rod 142 also isdisplaced at the same time to lock the differential mechanism 140.Further, the front wheel center clutch 340 is engaged. That is, the lawnmower makes a turn while driving the front wheels and one of the rearwheels.

[0057] When the front wheels 1 are maintained in the straight runningposture or steered by an angle not exceeding the predetermined steeringangle (swing angle of knuckle arms 30), the rear wheel differentialmechanism 140 is in unlocked state to be capable of differential action,and the two rear wheel clutches 27 are engaged. The front wheel centerclutch 340 is disengaged. As a result, the lawn mower is propelled onlyby the right and left rear wheels 2 (rear wheel drive).

What is claimed is:
 1. A lawn mower having selectively drivable wheels,comprising: dirigible front drive wheels; a pair of right and left reardrive wheels; a body frame; an engine supported by said body frame; amower unit vertically movably supported by said body frame forwardly ofsaid rear drive wheels; a change speed device for changing a speed ofdrive from said engine; a front wheel driving device for transmittingdrive from said change speed device to said front wheels; a left rearwheel driving device for transmitting drive from said change speeddevice to a right axle of said left rear wheel, and a right rear wheeldriving device for transmitting drive from said change speed device to aleft axle of said left rear wheel; rear wheel clutches associated withsaid left rear wheel driving device and said right rear wheel drivingdevice, respectively, for selectively connecting and disconnectingdrive; and an interlocking mechanism for operating said rear wheelclutches in response to a displacement to said front drive wheels from astraight running posture, to disconnect drive transmission to one ofsaid rear drive wheels moving along an inner track of turning of saidvehicle body, said interlocking mechanism having a link mechanismoperatively connecting a steering mechanism for steering said frontdrive wheels to each of said rear wheel clutches, and extending oversaid mower unit.
 2. A lawn mower as defined in claim 1, furthercomprising a grass catcher disposed in a rear position of said bodyframe, and a grass collecting duct for transmitting grass clippings cutby said mower unit rearward to said grass catcher, said grass collectingduct being disposed to pass between said right axle and said left axle.3. A lawn mower as defined in claim 2, wherein said change speed deviceis disposed laterally said grass collecting duct between said right axleand said left axle.
 4. A lawn mower as defined in claim 3, wherein saidleft rear wheel driving device extends from said change speed device tosaid left axle through a region at a lower level than said change speeddevice and said grass collecting duct, and said right rear wheel drivingdevice extends from said change speed device to said right axle througha region at a lower level than said change speed device and said grasscollecting duct.
 5. A lawn mower as defined in claim 3, wherein saidbody frame is formed of a pair of right and left band-like profilemembers extending in a fore and aft direction, said change speed deviceand said grass collecting duct being arranged between said right andleft band-like profile members.
 6. A lawn mower as defined in claim 3,wherein said link mechanism is connected to said rear wheel clutches ata lower level than said axles.
 7. A lawn mower having selectivelydrivable wheels, comprising: dirigible front non-drive wheels; a pair ofright and left rear drive wheels; a body frame; an engine supported bysaid body frame; a mower unit vertically movably supported by said bodyframe forwardly of said rear drive wheels; a change speed device forchanging a speed of drive from said engine; a rear wheel differentialmechanism with a differential lock function; a left rear wheel drivingdevice for transmitting drive from said change speed device through saidrear wheel differential mechanism to a right axle of said left rearwheel, and a right rear wheel driving device for transmitting drive fromsaid change speed device through said rear wheel differential mechanismto a left axle of said left rear wheel; rear wheel clutches associatedwith said left rear wheel driving device and said right rear wheeldriving device, respectively, for selectively connecting anddisconnecting drive; and an interlocking mechanism for operating saidrear wheel differential mechanism and said rear wheel clutches inresponse to a displacement to said front wheels from a straight runningposture; wherein said interlocking mechanism operates said rear wheeldifferential mechanism and said rear wheel clutches such that, when saidfront wheels are at a small steering angle, said rear wheel differentialmechanism is unlocked and said rear wheel clutches are both engaged, andwhen said front wheels are at a large steering angle, said rear wheeldifferential mechanism is locked and one of said rear wheel clutchesassociated with one of said rear drive wheels moving along an innertrack of turning of said vehicle body is disengaged.
 8. A lawn mower asdefined in claim 7, wherein said interlocking mechanism has a linkmechanism operatively connecting a steering mechanism for steering saidfront wheels to each of said rear wheel clutches, and extending oversaid mower unit.
 9. A lawn mower having selectively drivable wheels,comprising: dirigible front drive wheels; a pair of right and left reardrive wheels; a body frame; an engine supported by said body frame; amower unit vertically movably supported by said body frame forwardly ofsaid rear drive wheels; a change speed device for changing a speed ofdrive from said engine; a front wheel driving device for transmittingdrive from said change speed device to said front drive wheels through afront wheel differential mechanism; a rear wheel center clutch forselectively connecting and disconnecting drive from said change speeddevice; a left rear wheel driving device for transmitting drive fromsaid change speed device through said rear wheel center clutch to aright axle of said left rear wheel, and a right rear wheel drivingdevice for transmitting drive from said change speed device through saidrear wheel center clutch to a left axle of said left rear wheel; rearwheel clutches associated with said left rear wheel driving device andsaid right rear wheel driving device, respectively, for selectivelyconnecting and disconnecting drive; and an interlocking mechanism foroperating said rear wheel center clutch and said rear wheel clutches inresponse to a displacement to said front wheels from a straight runningposture; wherein said interlocking mechanism operates said rear wheelcenter clutch and said rear wheel clutches such that, when said frontwheels are at a small steering angle, said rear wheel center clutch isdisengaged, and when said front wheels are at a large steering angle,said rear wheel center clutch is engaged and one of said rear wheelclutches associated with one of said rear drive wheels moving along aninner track of turning of said vehicle body is disengaged.
 10. A lawnmower as defined in claim 9, wherein said interlocking mechanism has alink mechanism operatively connecting a steering mechanism for steeringsaid front drive wheels to each of said rear wheel clutches, andextending over said mower unit.
 11. A lawn mower having selectivelydrivable wheels, comprising: dirigible front drive wheels; a pair ofright and left rear drive wheels; a body frame; an engine supported bysaid body frame; a mower unit vertically movably supported by said bodyframe forwardly of said rear drive wheels; a change speed device forchanging a speed of drive from said engine; a front wheel center clutchfor selectively connecting and disconnecting drive from said changespeed device; a front wheel differential mechanism; a front wheeldriving device for transmitting drive from said change speed device tosaid front drive wheels through said front wheel center clutch and saidfront wheel differential mechanism; a rear wheel differential mechanismwith a differential lock function; a left rear wheel driving device fortransmitting drive from said change speed device through said rear wheeldifferential mechanism to a right axle of said left rear wheel, and aright rear wheel driving device for transmitting drive from said changespeed device through said rear wheel differential mechanism to a leftaxle of said left rear wheel; rear wheel clutches associated with saidleft rear wheel driving device and said right rear wheel driving device,respectively, for selectively connecting and disconnecting drive; and aninterlocking mechanism for operating said front wheel center clutch,said rear wheel differential mechanism and said rear wheel clutches inresponse to a displacement to said front drive wheels from a straightrunning posture; wherein said interlocking mechanism operates said frontwheel center clutch, said rear wheel differential mechanism and saidrear wheel clutches such that, when said front wheels are at a smallsteering angle, said front wheel center clutch is disengaged, said rearwheel differential mechanism is unlocked and said rear wheel clutchesare both engaged, and when said front wheels are at a large steeringangle, said front wheel center clutch is engaged, said rear wheeldifferential mechanism is locked and one of said rear wheel clutchesassociated with one of said rear drive wheels moving along an innertrack of turning of said vehicle body is disengaged.
 12. A lawn mower asdefined in claim 11, wherein said interlocking mechanism has a linkmechanism operatively connecting a steering mechanism for steering saidfront drive wheels to each of said rear wheel clutches, and extendingover said mower unit.